Solar cells are semiconductor devices that convert the energy of sunlight directly into electricity. They can be roughly divided into silicon-based solar cells, compound-based solar cells and organic-based solar cells based on the material to be used.
Silicon solar cells are subdivided into single crystalline silicon solar cells, poly-crystalline silicon solar cells, and amorphous silicon solar cells based on the phase of a semiconductor.
Additionally, solar cells are divided into bulk (substrate) solar cells and thin film solar cells based on the thickness of a semiconductor. Thin film solar cells are solar cells having a semiconductor layer thickness of several to several tens of μm.
Among silicon solar cells, single crystalline and polycrystalline solar cells are of a bulk type, and amorphous silicon solar cells are of a thin film type.
Meanwhile, compound solar cells are divided into bulk solar cells comprising GaAs (Gallium Arsenide), InP (indium phosphide), etc. of the III-V group and thin film solar cells comprising CdTe (cadmium telluride) of the II-VI group, CulnSe2 (CIS: copper indium diselenide) of the I-III-VI group, etc. Organic-based solar cells largely comprise organic molecular solar cells and combined organic and inorganic solar cells. Besides, dye-sensitized solar cells are comprised. All of which are of the thin film type.
As stated above, among various types of solar cells, bulk silicon solar cells having a high energy conversion efficiency and a relatively low manufacturing cost have been employed mainly for ground power applications in a wide range of applications.
However, with a rapid increase of the demand for bulk silicon solar cells in recent years, there is a tendency that the costs are increasing due to the shortage of materials. Thereupon, in order to develop techniques for lowering the cost of solar cells for large scale ground power applications and mass-producing them, there is a desperate need for the development of thin film solar cells which can reduce silicon material to 1/100 of the current number.
The large scaling of the thin film solar cells is easier than that of the bulk silicon solar cells. But, as the area of the thin film solar cells are larger, the efficiency of the converting energy is reduced because of the resistance of the transparent electrode.
The solution of the problem is a structure of the intergrated thin film solar cells. In the structure, the loss of the energy generated from the resistance of the transparent electrode is reduced because the transparent electrode is divided into plural strip shape and unit cells formed on the transparent electrode is electrically connected in series. The structure protects the large scaled solar cells from reducing the efficiency of the converting energy. In addition, practical high voltage is generated from one substrate in the structure and process for manufacturing a module is simple.
However, there are other problems in the structure and manufacturing process of intergrated thin film solar cells. Hereafter, the problems will be explained in detail.
FIG. 1 is a view showing a module structure of conventional integrated thin film solar cells. FIG. 2 is an example illustrating a laser patterning process for fabricating a transparent electrode, solar cell (semiconductor) layer and back electrode for the conventional integrated thin film solar cells.
As illustrated in FIG. 1, the conventional integrated thin film solar cells 1 are formed on a glass substrate or a transparent plastic substrate 10 by a plurality of unit cells 20 being electrically connected in series (hereinafter, “transparent substrate”).
Therefore, the module of the integrated thin film solar cells comprises a transparent electrode 22 formed in the shape of strips, being segmented (insulated) from each other, on top of the transparent substrate 10, which is an insulating material, a unit solar cell (semiconductor) layer 24 formed in the shape of strips by covering the transparent electrode 22, and a back electrode layer 26 formed in the shape of strips by covering the solar cell layer 24, and is constructed in a structure in which the plurality of unit cells 20 segmented (insulated) are electrically connected with each other in series. And, a back protective layer 30 made of resin is formed in a manner to cover the back electrode for the purpose of preventing and protecting the solar cells from electrical short-circuiting.
In order to manufacture the integrated thin film solar cells 1 of such a structure, a laser patterning method, a chemical vaporization machining (CVM) method, a mechanical scribing method using metallic needles and so on are generally used.
The laser patterning method is a technique of etching the transparent electrode 22, the solar cell (semiconductor) layer 24, the back electrode layer 26, etc. mainly by using a YAG laser beam. A concrete method of use will be described below.
As illustrated in FIG. 1, the transparent electrode 22 formed firstly on the transparent substrate 10 is etched in the atmosphere by using a laser beam, then the solar cell (semiconductor) layer 24 formed secondly is segmented (insulated) in the atmosphere by using a laser beam, and the back electrode layer 26 formed last is etched in the atmosphere by laser patterning, thereby electrically connecting the solar cells in series and forming an integrated solar cell.
Problems of such laser patterning method are to be noted.
First of all, the transparent electrode 22 formed on the entire top surface of the transparent electrode 10 is segmented (insulated) in the shape of strips having a pre-determined width by cutting by the laser patterning method as illustrated in FIG. 1. Then, the cut width is typically from 50 to several hundreds of μm.
The formation process of the solar cell (semiconductor) layer 24 to be formed next to the transparent electrode 22 is mostly performed in vacuum, while the laser patterning for cutting the solar cell (semiconductor) layer 24 is performed in the atmosphere, which disables a continuous process in vacuum, thereby deteriorating the operation efficiency of the manufacturing apparatus. As a result, such a process cannot help but act as a factor of increasing the cost of the solar cells. Further, since the substrate is exposed to the air for etching the solar cell layer 24, there may happen to a problem that the characteristics of the solar cell module are degraded due to adhesion of moisture and contaminants.
In the next step, a back electrode is formed in vacuum typically by a sputtering method, and then laser patterning is performed, thereby manufacturing an integrated solar cell. Such a process also may cause the aforementioned problems in process discontinuity and contamination. And, the cut width (ineffective area) between the solar cells 20, is widened, which is lost through two times of laser patterning for cutting the transparent electrode 22 and the solar cell (semiconductor) layer 24 and one time of laser patterning for cutting the back electrode 26 and connecting the solar cells in series, that is, a total of three times of laser patterning. Thus the effective area loss of the solar cells is increased. Moreover, the laser equipment for patterning is expensive, and a precision position control system is required for patterning at a precise position. Due to this, the manufacturing cost increases.
Meanwhile, the chemical vaporization machining method is a technique that cuts the solar cell (semiconductor) layer at once into a plurality of unit cells having a uniform width by locally generating an atmospheric plasma around line electrodes with a diameter of several tens of μm arranged in a grid pattern adjacent to the top of the substrate by using a gas of SF6/He or the like.
Such a chemical vaporization machining method has characteristics that the process time is short, the selectivity of films is superior, and damage to films is small as compared to the laser patterning method. Further, unlike the laser patterning method, etching is carried out in a vacuum state, thus it is possible to prevent the performance of the solar cells from being deteriorated due to an exposure of the substrate to the atmosphere, which is a problem of the laser patterning method, and reduce the manufacturing cost in comparison with the laser patterning method.
However, since etching has to be carried out at a precise position in conformity with the patterned transparent electrode, a precision position control system capable of precisely controlling a position in a vacuum apparatus is required. This emerges as a very difficult problem when it is intended to manufacture integrated solar cells using a large area substrate. Further, the gap that can be etched is about 200 μm to the smallest, which is greater than a (insulating) gap formed by the laser patterning method, and thus the loss of the effective area of the solar cells is increased.
As another etching method, a mechanical scribing method is comprised. This method enables bulk scribing corresponding to a required number of unit cells by using a plurality of metallic needles, and is higher than the laser patterning method in expandability and compatibility with high speed processing. Further, the apparatus and operation costs are the lowest relative to the above-described two methods.
In case of, for example, CIS solar cells, a CdS/CIS layer relatively softer than molybdenum (Mo) can be easily scribed by a scribing method, so it is widely used for the manufacture of CIS solar cells.
However, the existing mechanical scribing method is also limited to use with a solar cell (semiconductor) layer. Thus, there is a problem that laser patterning equipment and a precision position control apparatus or the like for precise position control are required so as to etch molybdenum (Mo) used as a back electrode and zinc oxide (ZnO) used as a front electrode.